Decreased colony spreading in Staphylococcus aureus with reduced vancomycin susceptibility

Staphylococcus aureus has been a significant bacterial cause of both healthcare-associated and community- acquired infections. Currently, vancomycin is one of the most drugs of choice used for treatment of Gram-positive infection. Thus, the S. aureus strains with reduced vancomycin susceptibility including vancomycin-intermediate S. aureus (VISA) and heterogeneous vancomycin-intermediate S. aureus (hVISA) have been reported increasingly. In this study, we aimed to compare the colony spreading ability, a phenomenon that S. aureus forms a giant colony on a soft agar surface, between vancomycin-susceptible and non-susceptible S. aureus. A total of 79 S. aureus strains, 44 vancomycin-susceptible S. aureus (VSSA), 27 hVISA, and 8 VISA isolates, were cultured on soft agar, and the colony spreading was investigated after 24 h incubation. Reference strains of VSSA ATCC29213, VISA strain Mu50, and hVISA strain Mu3 were included as positive and negative controls. Each bacterial isolate was tested twice on soft agar. The results showed that 61.4% of VSSA and 22.2% of hVISA were positive for colony spreading on soft agar, whereas none of the VISA isolates showed spreading colony. There were statistical differences between VSSA and hVISA (p = 0.0016) and VISA (p = 0.0014), but no difference between hVISA and VISA was found. These results suggest that S. aureus with reduced vancomycin susceptibility has a defect in colony spreading ability.